The invention relates to a surface-wave transducer devicexe2x80x94also called TAGsxe2x80x94and to identification systems in which it is used.
DE 42 17 049 discloses a passive surface sensor which can be interrogated in wireless mode. In this context, energy is transmitted by radio to the sensor from an interrogation device, the interrogation being done by means of chirped transmission signals. The sensor has transducers and reflectors. The reflectors reflect the chirp signal in a time-staggered sequence, so that the sensor returns a time-staggered chirp signal to the interrogation device. The reflection principle means that the surface-wave sensor described above (also called SAWxe2x80x94surface acoustic wavexe2x80x94sensor in the following text) has a very high insertion loss of the order of 50 dB, for example. When the chirp signal has been received by an SAW interdigital transducer as the input transducer, this transducer produces a surface wave which propagates on the substrate of the SAW arrangement in the direction of the reflectors. When an SAW is received, each individual reflector element returns a correspondingly reflected SAW to the SAW transducer which, for its part, produces an electromagnetic signal from the SAW. However, since each reflector element returns not just one SAW to the SAW transducer, but rather the reflector elements also reflect signals amongst one another, the reflections are inevitably relatively small, and a large part of the energy coming from the SAW transducer is lost in the reflector arrangement, so that only a low output power can be achieved. The problem of xe2x80x9cinternal reflectionxe2x80x9d from the reflectors cannot be avoided, in principle.
U.S. Pat. No. 5,734,326 discloses an SAW arrangement which comprises a synchronisation transducer and a multiplicity of tap transducers. When excited by a radio-frequency pulse signal, this SAW arrangement transmits a specific identification signal, so that the receiver in the interrogation unit can receive and process the transmitted TAG signal. A prerequisite of this processing, however, is that a filter in the interrogation unit is tuned exactly to the TAG from which the TAG signal is received. Accordingly, a prerequisite of the processing of a TAG signal is that the interrogation unit already knows which TAG identifier it has to search for, so that the filter in the interrogation unit can be preset accordingly for utilising the TAG signal. Hence, U.S. Pat. No. 5,734,326 discloses a TAG system which has very restricted options for application. If the interrogation unit does not know the identification of the TAG, it is impossible in practice to tune the filter in the interrogation unit to the TAG, because it would take too long to interrogate every conceivable variant.
A further disadvantage of the TAG disclosed in U.S. Pat. No. 5,734,326 is that the tap transducers are not programmable, which makes them considerably more complicated and expensive to manufacture.
If the TAG were intended to be capable of long interrogation range, the energy of the RF pulse signal would have to be increased considerably. To do this, however, the signal would have to have such a high output level that it would infringe the normal conditions of licensing authorities, which limit the signal forms and power spectra of transmitters to a specific level so that no undesirable interference is produced, such as is known from radar technology, although these are permitted there in a specific frequency range owing to important monitoring functions (air traffic monitoring).
Furthermore, xe2x80x9c1996 IEEE International Frequency Control Symposiumxe2x80x9d, pages 208-215 xe2x80x9cWireless Integrated System for SAW-Identification Marks and SAW-Sensor Componentsxe2x80x9d, and 1993 xe2x80x9cUltrasonic Symposiumxe2x80x9d, pages 125-130, xe2x80x9cProgrammable Reflectors for SAW-ID-Tagsxe2x80x9d, and xe2x80x9cIEEE Ultrasonics Symposiumxe2x80x9d, October 1998, xe2x80x9cSAW Delay Lines for Wirelessly Requestable Conventional Sensorsxe2x80x9d, and 1993 xe2x80x9cUltrasonics Symposiumxe2x80x9d, pages 1267-1270 xe2x80x9c2.5 GHz-Range SAW Propagation and Reflections Characteristics and Application to Passive Electronic Tag and Matched Filterxe2x80x9d and, in addition, IEEE xe2x80x9cUltrasonics Symposium 1998xe2x80x9d xe2x80x9cOn-Chip Correlationxe2x80x94A New Approach to Narrowband SAW Identification Tagsxe2x80x9d disclose further SAW TAG arrangements which likewise have a very high insertion loss (TAG attenuation) and accordingly also permit only a very short distance of 1.3 m that can lie between an interrogation device and the TAG in order that any communication at all is possible between the interrogation unit and the TAG. Furthermore, the known SAW TAG devices are characterised by a low bit capacity of approximately 20 bits.
In addition, various identification systems with coded passive transponders or SAW structures are disclosed in the following publications: DE 44 05 647 A1, DE 42 00 076 A1, DE 34 38 050 A1, U.S. Pat. No. 4,059,831, DE 34 38 053 C2, DE 43 36 504 C1, DE 43 10 610 A1, DE 34 38 052 C2, DE 43 36 897 C1, DE 31 02 334 C2, U.S. Pat. No. 4,096,977, U.S. Pat. No. 5,734,326, U.S. Pat. No. 5,374,863, U.S. Pat. No. 4,604,623, DE 34 38 051 A1 and DE 34 38 051 C2. However, all these publications disclose systems which have certain disadvantages; for example, either the interrogation distances are too short ( less than 1.3 m), the insertion losses are correspondingly too high or the solutions illustrated are virtually infeasible from the point of view of cost or permit only poor flexibility on account of a low bit capacity.
The object of the invention is to provide a surface-wave transducer device which has a low insertion loss, allows a considerably greater interrogation distance than with previously known TAGs, and has an increased bit capacity. The device should enable free programming that is simple to perform. The invention proposes a surface transducer device having the features according to claim 1. Advantageous developments are described in the subclaims.
The invention is based on the idea of providing a surface-wave transducer device with a combinational arrangement of a dispersive transducer device having a non-dispersive transducer device. A dispersive transducer device is capable of using a chirp signal to produce a surface-wave pulse signal, which is received by the non-dispersive transducer device, comprising n transducers, and is converted by this non-dispersive transducer device into a pulse code signal sequence which identifies the surface-wave transducer device. The pulse signal sequence is then transmitted by the surface-wave transducer device via an antenna and can be received by an interrogation unit within an identification system and evaluated on the basis of the identifier.
An SAW transducer device according to the invention is characterised by interrogation distances which are significantly longer than those known hitherto, has a very high bit capacity available and is very simple to program. The reason for the long interrogation distance and the correspondingly low insertion loss is that, once a signal is received from an interrogation device (interrogator), the SAW transducer device according to the invention converts the interrogation signal in the dispersive SAW transducer device into a time-compressed surface-wave signal which is received by the non-dispersive transducer device.
The non-dispersive transducer device also receives the interrogation signal and produces corresponding SAW signals which are received by the dispersive transducer. After receiving the signals, the said dispersive transducer converts the surface waves into corresponding electromagnetic signals. The identical transit times of the signals from the dispersive transducer device to the non-dispersive transducer device and vice versa mean that two identical signals are superimposed on one another at the output, so that the code signal to be transmitted by the TAG can have a higher signal power. Accordingly, the TAG""s response signal to an interrogation signal comprises two components; since both components are transmitted synchronously by the TAG, however, and both signal components can have the identical signal form, the TAG according to the invention can have a higher power efficiency and the TAG transmission signal has a considerably higher output level than the TAG signal in comparative SAW arrangements, the output level always being referred to comparable interrogator interrogation signals.
The surface-wave transducer device according to the invention advantageously does not have any reflectors, so that the insertion loss is considerably lower than with reflector arrangements. The insertion loss in known SAW TAGs with a reflector arrangement is about 50 dB. A surface-wave transducer device according to the invention and hence a TAG according to the invention allows the insertion loss to be reduced to a range of between approximately 30 and 35 dB, which drastically increases the interrogation range. The interrogation signal used is expediently a chirp signal which is converted into a time-compressed pulse signal on the SAW TAG according to the invention. In the case of a chirp interrogation signal, the TAG response signal comprises time-compressed pulse signals which are shifted in time, the modulation of the response signals depending on the programming of the n transducers within the non-dispersive transducer arrangement. If, by way of example, an individual transducer from the n transducers is not connected to the common bus lines, then this transducer also produces no response signal. Such a response can be regarded as being a logic xe2x80x9czeroxe2x80x9d.